Pressure-temperature Paths Research Articles

Thermal modeling of the southern Alaska subduction zone: Insight into the petrology of the subducting slab and overlying mantle wedge

In order to predict metamorphic conditions near the slab/mantle‐wedge interface and possible melting relations beneath the volcanic arc in southern Alaska, we have constructed a two‐dimensional kinematic thermal model of the southern Alaska subduction zone that includes an abrupt change in slab dip at 40‐km depth, time‐dependent convergence rate, and variable shear heating. The numerical results indicate that the shallow‐dipping slab segment in the southern Alaska subduction zone increases slab‐surface temperatures up to 180 ° C at depths < 40 km and up to 90 °C at greater depths. Shear heating may increase slab‐surface temperatures an additional 140 °C at depths < 40 km and 70 °C at greater depths. Calculated pressure‐temperature (PT) paths indicate that oceanic crust subducting beneath the southern Alaska forearc is undergoing blueschist‐facies metamorphism. Calculated slab PT paths do not intersect the wet basaltic solidus for shear stresses ≤ 10% lithostatic pressure, suggesting that observed arc magmas are not derived from partial melting of subducting oceanic crust. The mantle wedge adjacent to the subducting slab lies at suosolidus conditions, and at depths < 75 km, numerous hydrous phases are stable including serpentine, talc, amphibole, and chlorite. Low P wave velocities observed in the mantle wedge adjacent to the subducting slab probably reflect extensive hydration of mantle‐wedge peridotite caused by fluids liberated from the subducting oceanic crust and sediments. Calculated PT conditions in the core of the mantle wedge permit partial melting if H2O is present either as a free fluid phase or fixed in amphibole.

Ｇｉｂｂｓ ｍｅｔｈｏｄを用いた変成岩の温度圧力経路の推定

The Gibbs method is a technique to apply the change in mineral composition, which is preserved as compositional zoning, to decipher the pressure temperature path of metamorphic rocks. This method involves deriving a system of equations composed of a Gibbs-Duhem equa tion for each phase, stoichiometrical relations among the chemical potentials of phase components, and relationship between compositional term, dX, and other intensive variables. To solve these equations random mixing of substitution and constant molar entropy and volume are assumed. The continuous pressure-temperature path during garnet growth can be inferred by choosing garnet composition as monitor parameters.

Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania

U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth9s history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation.

Dating a Variscan pressure-temperature loop with staurolite

Staurolite with a complex internal metamorphic and structural relationship occurs in metapelite from the Silvretta thrust sheet (eastern Alps). Stepwise leaching of a core and rim fraction of a single staurolite crystal resulted in a total Pb-Pb isochron of 308 ± 28 Ma. The Variscan metamorphism has been confirmed by a further single bulk staurolite Pb-Pb age of 298 ± 42 Ma, which is in accordance with U-Pb ages of 306 ± 8 Ma and 304 ± 6 Ma, respectively, obtained from combined U-Pb leaching of the same crystals. Direct dating of the staurolite-in reaction isograd was therefore successful. Our results date the prograde part of a Variscan counterclockwise pressure-temperature path in the Silvretta thrust sheet. Proximity with white-mica Rb-Sr and K-Ar cooling ages suggests that the whole Variscan episode was fast and took place within a few million years.

Cooling and exhumation of the Western Betic Cordilleras, 40Ar/ 39Ar thermochronological constraints on a collapsed terrane

New 40Ar/ 39Ar data on amphiboles, muscovites, biotites and potassium-feldspars from different tectono-metamorphic units of the Western Alpujarrides (Betic Cordilleras, southern Spain) help to constrain the P-T-t evolution of this Alpine collisional belt. During an initial stage of plate convergence between Africa and Eurasia, the Alpujarride metamorphic rocks evolved along increasing pressure-temperature paths, locally reaching eclogitic conditions, but the timing of peak metamorphism is only constrained to be earlier than 25 Ma. In the interval of 25–22 Ma, the Alpujarride rocks underwent strong adiabatic decompression related to the collapse of the previously thickened crust. We propose that the main phase of synmetamorphic ductile deformation and thinning of the metamorphic pile was related to this extensional event. The last step is marked by fast cooling of the hot Alpujarride rocks below 600°C, resulting in a striking convergence of our 40Ar 39Ar determinations in the range 19–20 Ma. Cooling rates in the range 100–350°C/m.y. are indicated for this period, associated with exhumation rates of less than 3 km/m.y. We suggest that fast cooling took place primarily as a result of thermal relaxation of the abnormally steep geotherm resulting from extensional tectonics which has the effect of juxtaposing thinned rock bodies with contrasting temperatures along shear zones and faults. The main implication of this data set is that the Western Alpujarrides present a structural and metamorphic development which should be regarded as characteristic of “collapsed terranes”.

Role of horizontal thermal conduction and finite time thrust emplacement in simulation of pressure-temperature-time paths

Comparison of metamorphic thermal histories produced by (1) one- vs. two-dimensional conductive models and (2) instantaneous vs. finite emplacement time thrusting indicates that the simple 1D or instantaneous models applied by many petrologists generally produce pressure-temperature ( PT) paths with topologies similiar to those generated by 2D, time-transitive models. However, maximum temperatures attained by footwall rocks are significantly lower in 1D than in 2D conductive models, leading us to recommend that 2D thermal effects be incorporated into models whenever possible. PT paths produced by instantaneous and finite-duration thrust emplacement differ significantly during the synthrusting period, but the peak metamorphic conditions ( T MAX and pressure at T MAX) eventually attained by footwall rocks during erosional unroofing are the same within minimum analytical uncertainties ( ± 50 K, ± 100 MPa) on geothermobarometric data. Thus, if erosion begins only after the end of the thrusting event, instantaneous thrusting is a reasonable simplifying assumption that introduces relatively little inaccuracy into PT models devised to examine peak metamorphic conditions.

Diachronous burial and exhumation of a single tectonic unit during collision orogenesis (Sulitjelma, central Scandinavian Caledonides)

The Sulitjelma fold nappe represents part of a Caledonian marginal basin obducted during the closure of the Iapetus Ocean and collision between Laurentia and Baltica. Metamorphic pressure-temperature (P-T) paths indicate that various parts of the Sulitjelma fold nappe followed characteristic clockwise P-T paths involving prograde burial followed by prograde exhumation and then near-isothermal exhumation prior to cooling. Geochronological results indicate that foreland rocks followed this general P-T path before more hinterlandward rocks, such that foreland rocks underwent exhumation while hinterlandward rocks were still being buried. This is consistent with the fold nappe passing through a collisional orogen; burial and ultimately prograde metamorphism were terminated sequentially by exhumation as a given part of the fold nappe came into contact with the lower plate, basement ramp (Baltica) and was driven upward. Burial, heating, exhumation, and cooling were thus diachronous within a single tectonic unit.

三波川変成作用から見た沈みこみ帯の進化

Evolution of pressure-temperature paths and fluid compositions during the Sanbagawa prograde metamorphism was reviewed on the basis of chemical characteristics and zonal structures of sodic pyroxene, amphibole, garnet and phengite. Zonal structures of sodic pyroxene, garnet and phengite imply that (1) dP/dT of prograde P-T paths of individual rocks decreased with increasing metamorphic grade and (2) dP/dT just before metamorphic climax might have been negative in some high-grade rocks. Changes of amphibole composition indicate that the prograde P-T path recorded in the higher-grade rocks was situated on the higher-temperature side of that of the lower-grade rocks and on the lower-pressure side of the metamorphic field gradient (1.4-1.8kbar/100°C). The systematic change of prograde P-T path implies an increasing dP/dT during continuous subduction by continuous cooling of the surrounding mantle. These features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a non-steady-state geotherm. Xco2 [=CO2/(CO2+H2O)] value of metamorphic fluid seems to increase with increasing metamorphic grade, and reaches>0.2 in some higher-grade rocks. The increasing Xco2 might have been promoted by oxidation of carbonaceous material caused by mechanical mixing of metapelites and Fe2O3-rich metabasites. Differences in petrological and age data between the Sanbagawa high-grade rocks and pebbles of high-grade in the Eocene Kuma Group in central Shikoku were also briefly reviewed to discuss evolution of subduction zone in the southwest Japan of Jurassic-Cretaceous age.

Melt Segregation and P-T Path of Mantle Melting

Chemical composition of the mantle which is undergoing melting can change as melt segregation proceeds, which affects the pressure-temperature path of the melting. The effects can be important when fractional-type melting occurs, because the melt is efficiently isolated from the residue in terms of chemical interaction. In this paper we investigate the critical conditions for chemical and thermal isolation caused by melt segregation and the effects of the isolation on mantle melting.

Correlation by Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window, Eastern Alps

In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhulle (USH) and Lower Schieferhulle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67 −0.13 +0.19 mm/million years and 0.88 −0.19 +0.34 mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7 −0.7 +1.2 ×10-14 s-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at ∼30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4 −2 +3 mm/year and 2±1 mm/year, respectively.

The Saxonian Granulites: a metamorphic core complex?

The brachyanticlinal structure of the Saxonian Granulite Complex was formed, or at least overprinted, by large-scale crustal extension, which is documented in an extreme reduction of the metamorphic profile and a pressure-temperature path with nearly isothermal decompression. the granulite is situated within a south-west-northeast trending belt characterized by basaltic volcanism and an intrabasinal, sediment-producing rise, both active in Frasnian times. These features are taken to reflect at least part of the extension which has exhumed the high grade rocks; however, Cambro-Ordovician rifting possibly initiated the process. The Frasnian palaeogeography and geophysical data suggest that the axis of the granulite dome extends at least 150 km towards the south-west. The overall setting strongly resembles the metamorphic core complexes of the western USA, but the exhumation of rocks preserving evidence of metamorphic pressures of 10–12 kbar requires much higher extensional strain, possibly due to the longer period of stretching. Early Carboniferous compression and minor post-collisional extension (Late Carboniferous to Permian) have not significantly modified the earlier geometry. U-Pb and Sm-Nd data ranging between 380 and 340 Ma are interpreted to record retrogression during uplift rather than peak metamorphism.

Monomineralic Glomerocrysts: Textural Evidence for Mineral Resorption during Crystallization of Igneous Rocks

Monomineralic aggregates or glomerocrysts are a distinctive feature of many igneous rocks. Those with crystals joined along prominent euhedral crystal faces form by synneusis while the mineral is crystallizing. Other glomerocrysts, comprised of crystals joined along irregular, embayed margins, arguably formed while the crystals were being resorbed. A model for glomerocryst formation during mineral resorption is described. During resorption, a boundary-layer melt, which envelops the dissolving crystals, is produced. When two or more dissolving crystals come into close proximity, the intervening portion their respective boundary-layer melts becomes isolated from chemical communication with the surrounding granitic liquid. Contributions from both dissolving crystals to this isolated portion of the boundary layer melt creates a melt composition that eventually becomes supersaturated with respect to the dissolving crystals. This results in crystallization of new material from the intervening melt that bonds the partially resorbed crystals together. Any process that causes mineral resorption, such as a change in intensive parameters (e.g., P, T, , can lead to glomerocryst formation. Such glomerocrysts can be used to define pressure-temperature paths of igneous rocks that are compatible with mineral resorption.

The importance of blueschist → eclogite dehydration reactions in subducting oceanic crust

Research Article| May 01, 1993 The importance of blueschist → eclogite dehydration reactions in subducting oceanic crust SIMON M. PEACOCK SIMON M. PEACOCK 1Department of Geology, Arizona State University, Tempe, Arizona 85287-14042Institut für Mineralogie und Petrographie, ETH Zentrum, CH-8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar GSA Bulletin (1993) 105 (5): 684–694. https://doi.org/10.1130/0016-7606(1993)105<0684:TIOBED>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation SIMON M. PEACOCK; The importance of blueschist → eclogite dehydration reactions in subducting oceanic crust. GSA Bulletin 1993;; 105 (5): 684–694. doi: https://doi.org/10.1130/0016-7606(1993)105<0684:TIOBED>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure-temperature (P-T) paths with a model of metabasalt phase equilibria. In steady-state subduction zones with high rates of shear heating, the upper parts of the subducting oceanic crust progress through the greenschist → amphibolite → granulite → eclogite facies, whereas lower parts of the subducting oceanic crust progress through the blueschist → eclogite facies. In steady-state subduction zones with moderate rates of shear heating, most of the subducting oceanic crust passes through the blueschist → eclogite transition. In steady-state subduction zones with low rates of shear heating, the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km. For oceanic crust containing 1-2 wt% H2O, dehydration will not begin until the onset of eclogite- or amphibolite-facies metamorphism, depending on the P-T path. For many subduction zones, the most important dehydration reactions in the subducting oceanic crust occur at the blueschist → eclogite facies transition associated with the breakdown of lawsonite (or clinozoisite), glaucophane, and chlorite. Large amounts of H2O released by blueschist → eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Retrograde deformation within the Carthage-Colton Zone as recorded by fluid inclusions and feldspar compositions: tectonic implications for the southern Grenville Province

The Carthage-Colton Zone (CCZ), located in the northwestern portion of the Proterozoic Adirondack terrane, forms the boundary between the Adirondack lowlands and highlands and is characterized by textures indicative of ductile deformation. Samples from three rock types have been collected from this zone: (1) pyroxene bearing syenite gneisses of the Diana igneous complex; (2) paragneiss samples from near the northwestern boundary of the Diana complex; (3) a quartz-rich metasediment. Feldspar geothermometry performed on the Diana metasyenites shows that porphyroclasts are relict igneous grains that formed atT≈900° C, while dynamic recrystallization occurred at temperatures as low as 470 to 550° C as shown by the compositions of feldspar neoblasts. All samples examined in this study from the CCZ contain a suite of CO2-rich fluid inclusions that are distinctive both texturally and in their microthermometric behavior (Th=-27.7 to-7.1° C) as compared to CO2-rich Adirondack fluid inclusions that do not lie within this zone (Th=-45.9 to +31.0° C). The results from fluid inclusion microthermometry and feldspar geothermometry restrict the conditions of dynamic recrystallization to temperatures and pressures of 400 to 550° C and 3 to 5 kbar. The retrograde pressure-temperature path must pass through these conditions. Similar fluid inclusion results have been obtained from the Parry Sound Shear Zone (PSSZ) which is a Grenvillian shear zone that is located in Southern Ontario (Lamb and Moecher 1992). However, the inferred retrogradeP-T paths for these two areas, the CCZ and the PSSZ, are different and this difference may be a result of late deformation along shear zones that are located between the two areas.

Pressure, Temperature, and Structural Evolution of West-Central New Hampshire: Hot Thrusts over Cold Basement

Pressure-temperature (P-T) paths have been calculated from pelites and amphibolites of several major Acadian structures in west-central New Hampshire by using both inclusion thermobarometry and differential thermodynamics (the Gibbs method). P-T paths calculated for rocks exposed in the Orfordville and Bronson Hill anticlinoria are ‘clockwise’ and show 1–2.5 kb of exhumation with 30–100 °C of heating. Because this type of path is characteristic of the lower plate of overthrust terranes, these rocks are interpreted to be (para)autochthonous. P-T paths for rocks exposed in an intervening synclinorium (the Hardscrabble synclinorium) show isothermal loading of 1–3 kb followed by possible isobaric cooling. This behavior is characteristic of rocks occupying a middle-plate structural position within a multiple thrust package, and so these rocks are interpreted to be allochthonous. The interpretation that the Hardscrabble rocks are allochthonous differs from previous models, but better explains the petrologic data and is consistent with the stratigraphic and structural data on which other models have been based. Correlation of the P-T paths with deformational events through kinematic and textural analysis indicates that during nappe stage deformation, the synclinorial rocks were transported westward, and that the anticlinorial and synclinorial rocks were buried to depths of 25–30 and 20–25 km respectively. The exhumation with heating recorded by the anticlinorial samples occurred during the dome stage of deformation, and differentially uplifted the anticlinorial rocks relative to the synclinorial rocks; this differential uplift may have been accommodated through reactivation of early thrust faults with normal movement sense. P-T paths of the Hardscrabble synclinorium rocks are suggestive of a relatively elevated initial geothermal gradient for their pre-nappe source terrane, which is interpreted to have been between the Kearsarge-Central Maine basin and the Bronson Hill parautochthon.

Hydrothermal ore-forming processes in the light of studies in rock-buffered systems; II, Some general geologic applications

The experimental metal solubilities for rock-buffered hydrothermal systems, reported by Hemley et al. (1992), provide important insights into the acquisition, transport, and deposition of metals in real hydrothermal systems that produced base metal ore deposits. Water-rock reactions that determine pH, together with total chloride and changes in temperature and fluid pressure, play significant roles in controlling the solubility of metals and determining where metals are fixed to form ore deposits.Hydrothermal systems circulate fluids and heat, and the transport path of a hydrothermal fluid is likely to lie somewhere between an adiabatic (no heat loss to adjacent rocks) and a geothermal (complete thermal equilibrium with adjacent rock) path. The transport path of the hydrothermal fluids emanating from, or circulating near, deep-seated crystallizing plutons can be approximated by a quasi-adiabatic pressure-temperature path. In such a quasi-adiabatic setting, the pressure effect on rock-buffered metal solubilities is significant and allows metal transport over long distances because the trend of decreasing metal solubility with decreasing temperature is compensated by the trend of increasing metal solubility with decreasing pressure. The high-temperature portion of a quasi-adiabatic hydrothermal system will tend to leach metals from the rock and fix K and Na in feldspars. The source of the extracted metals may be late-stage magmatic melt, trace metals distributed in the lattice of silicate minerals destroyed during rock metasomatism, and/or small amounts of base metal sulfides disseminated throughout a given rock.Deposition of metals in hydrothermal systems occurs where changes such as cooling, pH increase due to rock alteration, boiling, or fluid mixing cause the aqueous metal concentration to exceed saturation. Relative metal transport concentrations, the availability of sulfur, the disposition of the saturation surfaces relative to each other, and the interplay of these variables through time are the major factors controlling the pattern of metal deposition (and nondeposition).Metal zoning results from deposition occurring at successive saturation surfaces. Zoning is not a reflection simply of relative solubility but of the manner of intersection of transport concentration paths with those surfaces. The experimental results are consistent with the typical outward zonation of Cu-Zn-Pb observed in porphyry coppers, Butte-type base metal vein deposits, skarns, and massive sulfides. Implications to mineralization patterns in Mississippi Valley-type, sedimentary Cu, and other low-temperature deposits are also of interest, with due recognition of the greater uncertainty regarding speciation and attainment of equilibrium in those environments. In such deposits, a probable outward zoning of Cu-Zn-Pb-Fe is suggested from the results.Saturation surfaces will tend to migrate outward and inward in prograde and retrograde time, respectively, controlled by either temperature or chemical variables. This, in turn, gives rise to zone migration and, where one zone encroaches on another, the appropriate apparent paragenetic relations. Such textural implications are incorrect, however, unless viewed within the context of the overall mineralization process. Additional controls bearing on metal precipitation sequence and coprecipitation, the presence or absence of zoning, reversals in zoning, sulfidation state, and timing relations between alteration and metallization are implicit in the results and are discussed.

Exhumation of coesite-bearing rocks in the Dora Maira massif (western Alps, Italy)

Coesite-bearing, very high pressure rocks in the southern Dora Maira massif, western Alps, Italy, emerged from a depth of 100 km during Cretaceous time. These rocks are now covered only by a thin fraction of the synmetamorphic overburden. Tectonic contacts above the coesite-bearing rocks are retrograde ductile shear zones that operated with a normal sense of motion: they cut out thick crustal sections from within the tectonometamorphic pile and caused the condensation of the Eoalpine baric and thermal structure. The result of this was the lateral dispersion of the Eoalpine orogenic wedge and the facilitation of the unroofing of the very high P rocks. Extensional unroofing is consistent with the absence of deeply cutting Cretaceous erosion, and with pressure-temperature paths that show that the very high P rocks cooled during decompression. The very high P rocks reached relatively shallow crustal levels by ca. 40 Ma, and the attenuated sequence overthrust a more external unit. These indicate that the exhumation of the very high P rocks took place within an overall plate onvergence setting, prior to the final suturing of Europe and Adria.

How well established is isobaric cooling in Proterozoic orogenic belts? An example from the Arunta inlier, central Australia

Orogenic pressure-temperature paths can only be deduced from metamorphic textures if they formed during a single metamorphic event, rather than recording the effects of two or more separate events. Erroneous tectonic models are likely to result if textures formed in several events are ascribed to one event. We illustrate this problem for the Arunta inlier in central Australia, which has been considered to be a good example of an isobarically cooled terrain. Rocks of the Anmatjira Range in the central Arunta inlier, which has been described in terms of isobaric cooling, actually record the effects of two low-pressure, granulite facies, metamorphic events of similar peak metamorphic grade, M 1 and M 2 , that are separated in time by a period of basin development and deposition of a succession of sedimentary rocks. Correlations of stratigraphic, structural fabric, and metamorphic texture data allow the effects of M 1 and M 2 to be distinguished. Without this fine control, the textures formed by superposition of M 1 upon M 2 , considered as a manifestation of one event, could easily be interpreted as indicating isobaric cooling, because the overprinting assemblages are of similar pressure but slightly lower temperature than the original assemblages. We call this apparent isobaric cooling; in fact, we now recognize textures reflecting decompression that formed immediately following the peaks of both M 1 and M 2 . It is now a matter for conjecture whether isobaric cooling is more apparent than real in other terrains for which it has been recorded.

Constraints on the Tectonic Evolution of the Northwestern Himalaya from Geochronologic and Petrologic Studies of Babusar Pass, Pakistan

Petrologic, geochronologic, and structural studies of Indian-plate rocks in the footwall of the Main Mantle Thrust indicate that peak metamorphism occurred in thrust imbricates well before the final assembly of the Kohistan island-arc with the Indian plate. As the northern margin of the Indian plate was thickened by this early thrusting, the metamorphic rocks evolved along increasing pressure-temperature paths and, in some regions, reached temperatures sufficient for partial melting. It is difficult to constrain the timing of this early deformation and metamorphism, but $$^{40}Ar/^{39}Ar$$ hornblende ages and U/Pb ages of crosscutting leucogranite dikes suggest that in most areas peak metamorphism was earlier than ~40 Ma and in some areas earlier than 50 Ma. After peak metamorphism, in the interval from ~40 to 25 Ma, rocks of the Indian plate continued to be thrust southward, creating sharp metamorphic discontinuities at thrust boundaries. From 25 to 20 ma rocks of the Indian plate underwent rapid cooling, and data suggest that this rapid cooling occurred over large regions of the northern margin of the Indian plate, along as much as 300 km of the Main Mantle Trust. We tentatively suggest that the rapid cooling resulted from tectonic denudation of the Indian plate as Kohistan slid northward along later normal faults. Between 15 and 20 Ma, faulting ceased and the terranes were locked into their present structural position; since then these terranes have simply undergone uplift and erosion. North of the Babusar region, in the vicinity of Nanga Parbat, tectonic activity along the Main Mantle Thrust continued to the present. In this region, the Indian plate rocks are presently being thrust over the adjacent Kohistan terrane.

Effects of diffusional modification of garnet growth zoning on P-T path calculations

The effect of intragranular diffusion on chemical zoning in garnet and on P-T paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth. Syn-and post-growth diffusion of Mg−Fe−Mn−Ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635°C. Compositions from zoned garnet, as modified by diffusion, hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure-temperature (P-T) paths from isobaric and polybaric model histories. P-T path calculations, based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635°C, show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths. Retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path. The magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition. The effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating-only models that reached peak temperatures approximately 15–30°C higher. Compositions of garnets with radii less than 1 mm, that reached actual peak temperatures of 605°C along temperature-time histories characteristic of regional metamorphism, experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories. Path distortions were significant at heating/cooling rates less than 10°C/Ma, but rapidly diminished for rates faster than this. In polybaric simulations diffusion effects were least noticeable when the actual pressure-temperature conditions changed in a clockwise sense (i.e., convex to higher P and higher T), but apprecciable modification was seen in path models that underwent counterclockwise changes in P and T. Reequilibration of garnet rim compositions occurred during cooling on all paths, and temperature maxima obtained from garnet-biotite geothermometry underestimated actual peak conditions by 40 to 70°C. Calculations suggest that P-T path trajectories calculated from garnets of at least 1 mm size, and that experienced actual thermal maxima below 585°C, are not likely to be distorted by diffusional effects during regional metamorphism. However, P-T path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history. The partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling, seriously complicating the task of quantitatively estimating diffusion effects on path calculations.